Harvard H407 CB RX audio problem

[Techman]

Originally Posted by Damo666

Can anyone suggest a decent entry level frequency counter to look out for, and that way I can create a "followed search" on eBay & be instantly notified if something is listed?

You may be able to pick one up at one of these junk ham radio auction type sales. I bought a couple a few months back for a quid each, untested! I may start a thread on them for the sake of interest and information. One of them worked and the other needed some 'attention' to get it working properly. Remember that any old counter that you buy may be out of calibration.

[Damo666]

Will do Techman.

I'll have to fetch the rig downstairs I think, as the strong signals necessary to show the problem tend to only come on in the late evening, by which time I've had to vacate the bedroom it's in due to my toddler sleeping in there. He's a light sleeper.

The aforementioned, in addition to various other commitments is the reason this is taking so long to get to the bottom of.

I'll fetch it downstairs right now before I forget and keep it switched on permanently. There's no access to the outside vertical here, but I do have a very long length of wire which should bring in a decent signal to trigger things off.

[G0HZU_JMR]

My advice on the way forward would be to slow down and try and fix this radio in slow time. Even if that means fixing it later in the year. Otherwise you could end up throwing loads of time and money at something you don't really need once it finally gets fixed.

My advice on a frequency counter would be to be patient and buy an old used counter from one of the big names like Racal or Philips or maybe HP. Definitely buy something with an external reference option on the rear panel.

Otherwise there are new entry level counters available on ebay for similar money that you can buy straight away. Eg the VC3165 seems to be very popular. But I wouldn't want one. I'd rather take a chance and buy a top brand used counter. The prices seem to vary, I wouldn't want to pay more than £50 for a used counter if it was just going to be used for old school CB and ham stuff. If you are patient you will find one for much less money.

[G0HZU_JMR]

I did the pin 7 tests this afternoon and it showed 6.11V when cold although this wandered from 6.07 to 6.2V but once it was on for a while it was typically 6.25V with no signal. Note that I also set my PSU to give 12.67V at pin 11 of the LA1230 to match the homebase radio reading here.

I did try optimising the quad demod coil adjustment and this thing is very critical on the adjustment. It only has to be out by a tiny amount to wreck the audio. Note that I did this with a two tone test waveform to look at 2nd and 3rd order distortion on pin 6 (on an AF spectrum analyser) as I adjusted the coil.

I also put the tip of my little finger on the centre of the LA1230 chip and held it there and my thermal camera is correct. The centre of the chip runs quite hot. Much hotter than you would think! I assume that mine is OK because it seems to be working fine in FM receive. These things were never that great in terms of 'fidelity' (despite the name on the front panel ) but I think it is working as I remember back in the day. Not as harsh as a 134 radio but other radios will be nicer to listen to.

[Damo666]

Jeremy,

When I was testing the voltage of pin 7, I found I was getting fluctuations in voltage readings like yours when I was using the chassis as the negative. However, I was getting different but solid readings when I was using transformer cans as the negative test point, if that makes sense? I found this odd, as the transformer cans are still connected to ground - unless my DMM is prone to slight RFI?

Anyway, in a nutshell, it seems that the dropping voltage of pin 7 on mine is presumably something to do with my problem - because your readings are constant as opposed to dropping.

I've ordered some 100uF Cap's, because they're handy & I have none, so I'll replace this one connected to pin 7 when I get them - as maybe the cap is leaky?

I'll hold off on the LA123O as you've all suggested & take things slow - besides, it's been pretty quiet on the CB these last couple of days, and no strong signals have been apparent which I really need.

[Techman]

If you've got another room to try the radio in, you could do what I sometimes do and that is to connect another CB radio to your aerial, and with it set on low power on a blank channel, lock the mic on with a 'broadcast' radio set on a speech only type station placed within normal speaking distance of the mic. Then have the radio on test in another room, or even the same room for this particular test if you can put up with a bit of audio feed back, with no aerial connected and on the same channel as the other radio you'll get a good +30 signal for your test.

[Damo666]

Techman,

That's a very good idea you suggested, so I'll do just that when I get back home later this week. In fact, I'll get my wife to modulate the mic' on my behalf.

Thank you for the idea.

[Damo666]

Sorry about the lack of updates recently, but I was involved in a Car accident, and although not major I was pretty much unable to move about much all week due to a back issue.

I'll get back into the swing of things with this problem soon, and update the thread in due course.

[SiriusHardware]

Sorry to hear about the 'Dunsh', and of course we wish you a speedy recovery.

I was in the loft today and found that Mustang carcass I knew I had. It's been well cannibalised and was a scrapper before it ever came to me but it does still appear to have its LA1230 in situ.

If you get to the point where you think you need one I'd be happy to unsolder it for you but there's no way of knowing that it wasn't the original fault on this radio, of course.

[Damo666]

Still haven't got around to attending to this distorted audio issue, but I'm back to normal now so should seeing to it should be imminent.

For what it's worth, I recently purchased a Black star Meteor frequency counter from a friend off the CB for a princely sum of £20 - so maybe this will come in handy for fault finding on this rig?

The counter covers 0 > 600 Mhz, reads 8 or 9 digits, and with a stubby antenna connected to one of the counters BNC sockets, I can pick up & read a 1W PMR handheld from about 10 feet away with ease.

[Biggles]

I have had a Black Star Meteor 600MHz counter from new when they were still in production and that's quite a few years ago. It's a really good piece of kit and has proven to be very reliable. You can run it from nicads too, making it portable. They sometimes need a tweak of the pot on the front to get reliable readings at low frequencies due to the filter, but otherwise can't fault them.
Alan.

[Damo666]

Hi Alan,

Yes, it's not bad at all. Seems pretty rugged, too.

I'm currently running it on 6 x Alkaline C cells, but I can see it being quite power hungry due to all the LED displays - so I'm going to source a relevant adaptor for it.

It'll definitely come in handy at some stage as the handheld counter I have only covers down to 50 Mhz & has no BNC inputs.

[Biggles]

I may be wrong but I think nicad batteries can be recharged while running it on the adapter. To that end you may need to be careful not to unintentionally charge your alkaline cells if running on mains power. I probably have an instruction book for it somewhere if you need info but they are pretty straightforward to use. I was always leaving mine switched on by mistake when running on batteries and running them flat so now it only gets used on mains power via the adapter, which has to supply a bit of current, as you correctly say, I think it uses quite a bit of juice.
Alan.

[KeithsTV]

I've had a Meteor 600 from new and have posted copies of the user and service manuals here

https://www.vintage-radio.net/forum/...ad.php?t=48608

The power supply is 9V @ 600mA.

Keith

[Damo666]

I wanted to know the adaptor specification, so many thanks for the info', Keith.

I'll search around & buy one, because the cost of NiMh C cells is pretty astronomical and is false economy considering the little use the counter will get.